In today's electrical supply systems, there are occasions when alternate sources of electrical power are necessary or desirable. For example, the capability of switching from utility power to emergency generator power is extremely important for many businesses, hospitals and industries, as well as residential dwellings.
In certain applications, it is desirable for separate electrical circuits, or separate groups of electrical circuits, to be arranged so that when one circuit or group of circuits is switched to a conductive state, another circuit or group of circuits is switched to a non-conductive state in an alternating fashion. In one arrangement, it may be desirable to alternately switch a common load between separate power sources, so that as one power source is disconnected from the load, the second power source is connected after a negligible delay so as to limit interruption of electrical power to the common load. In order that the desired period of alternate switching may be minimized, a need has been recognized to employ a coupling mechanism which functions to switch one circuit or group of circuits OFF as the other circuit or group of circuits is switched ON.
A known electrical load center includes an electrical panel with a pair of transfer-type switches that selectively control the supply of electrical power from a standard utility 125/250 VAC service and a generator power supply, respectively. The typical generator includes a pair of “hot” or electrically-charged conductors, and a “neutral” or return conductor.
There are instances in which it is convenient to use a 125/250 VAC “bonded-neutral” generator (which includes a neutral conductor and a safety grounding conductor that are internally bonded together) for the purpose of powering structures or dwellings. A building is typically fed by a standard utility 125/250 VAC service that includes a neutral bus connected to a safety grounding bus, and the safety grounding bus conductor is connected to a grounding rod or net. Using the two-pole switch configuration described above, the pair of “hot” conductors from the generator are connected to the appropriate poles of the two-pole transfer switch, the neutral conductor of the generator is non-switchably connected to the neutral bus conductor of the electrical panel, and the safety grounding conductor of the generator is non-switchably connected to the safety grounding bus conductor of the electrical panel.
However, this configuration, known as a “non-separately derived system”, has drawbacks when used with bonded-neutral generators. For example, assume the transfer switch of the above-described system configuration is in the ON position such that the generator is supplying electrical power via the pair of “hot” conductors to a common load in the dwelling. Electrical current flows from one of the “hot” conductors of the generator through the transfer-type switch in a known manner so as to power the electrical load in the dwelling. The electrical current then returns via the neutral conductor of the load to the neutral bus conductor of the load center. A first portion of electrical current then flows from the neutral bus conductor of the building back to the neutral conductor of the generator, thus completing the circuit path. A remaining portion of electrical current flows from the neutral bus conductor of the building to a neutral-to-ground tie bar at the electrical panel, through the grounding bus conductor, back through the safety ground-to-neutral bonding conductor of the generator, and then through the neutral conductor of the generator, completing another circuit path. It is this undesired dual path for electrical current to follow back from the load to the generator that creates a problem.
Rather than the dual path current flow described above, such a power system should be electrically grounded in such a manner that prevents a flow of electrical current via the neutral conductor of the building back to the safety grounding conductor of the generator, in all situations except for an electrical power fault (q.v., Article 250 of the National Electrical Code). The safety grounding conductor is expected to be pristine or absent of the normal flow of electrical current, and instead is to be used to conduct electrical current safely to ground only when there is an electrical fault occurrence. Thus, system configurations of this type (known as “non-separately derived” systems”) are undesirable because such configurations allow a normal flow of electrical current to pass via the neutral conductor of the building to the safety grounding conductor of the generator. Another drawback of above-described known system configurations is that the flow of electrical current to the safety grounding conductor of the generator has been known to trigger a ground fault circuit interrupter at the generator. When triggered, the ground fault circuit interrupter will de-energize the “hot” conductors of the generator and prevent the supply of electrical power to the service bus conductor of the building.
In an attempt to address the drawbacks described above, a “separately-derived” system configuration can be utilized. This system configuration uses one or more transfer switches to control switching connection of the neutral conductor and the pair of “hot” conductors of a “bonded neutral” generator. Again, for purposes of example, assume the transfer switches are initially positioned such that electrical current flows from one of the pair of “hot” electrical conductors of the generator to the common load of the building. Specifically, the electrical current flows from the “hot” conductor of the generator through the transfer-type switch in a known manner, and to the electrical load. The electrical current then returns via the neutral conductor of the electrical load. However, instead of electrical current flowing directly to the neutral bus conductor of the dwelling, the flow of electrical current is first routed by a separate neutral switch assembly to the neutral conductor of the generator, thus completing the circuit. Thereby, this system prevents the undesired flow of electrical current through the generator safety ground-to-neutral bonding conductor and back to the generator neutral conductor, as noted previously.
In certain configurations, one or more standard linked circuit breakers or switches are used to selectively transfer power supplied to an electrical panel between a primary (typically utility) power supply and an auxiliary (typically generator) power supply. In configurations such as this, there is a need to develop a satisfactory system for connecting the neutral switch contacts of the circuit breaker or switch to the neutral bus bars or lugs of the panel to which the circuit breaker or switch is connected.